Over the past few decades, human activities such as industrialization and urbanization due to rapid development in Malaysia have increased the magnitude of pollution, and hence increase thickness of sediment deposits in the exiting drainage system of the country. This phenomenon has reduced the effectiveness of the urban drainage system that is built to cater and convey surface runoff. Therefore, a study on sediment size characteristics in existing concrete drain is essential at a preliminary stage to develop a suitable solution for the maintenance of a conventional drainage system and hence avoids the possibility of flash flood due to excessive sediment deposition in the drains.

Study Sites:

Surveys had been carried out to determine suitable sites for the study. As the result, eight sites in urban area in Malaysia had been selected to determine the sediment size characteristics in existing concrete drains. The sites are located at Alor Setar, Ipoh, Johor Bahru, Kota Bahru and four sites in Seberang Perai, i.e. Chain Ferry, Bandar Seberang Jaya, Taman Inderwasih Perai and Taman Perindustrian Bukit Tengah. (Figure1 (a) and (b)).

Recent results of sediment deposition data collection are presented in Table 1 and Figure 4. The result shows that sand and gravel are the major components that are found in the sediment samples. This indicates that the sediment size distribution in study sites is mainly non-cohesive. The average size of sediment in eight study sites varies from 0.5 to 1.2 mm

Gross Pollutant Trap (GPT) is an engineered sediment trap designed to catch and remove litter, debris, and coarse sediment from runoff. The main function of GPT is to keep coarse sediment out of ponds, protecting the vegetation at the head of the pond from the smothering effects of sediment. It may also be used as the pretreatment for flow into a pond or wetland to confine the area of deposition of coarse sediments. Besides coarse sediment, traps will also provide some reduction in other pollutants with the installation of an additional device such as the removal of particulate nutrients, trace metal, oil and grease, as well as reduction of bacteria and dissolved oxygen-demanding substances.

In the project, the proposed GPT is designed to trap sediment and gross pollutants of stormwater coming from roofs, yards, roads or lawns in an urban area. Additionally, the proposed GPT will include pollutants separation from flows by the energy of the water flow (self-cleansing method).

Gross Pollutant Trap Prototype Design

The GPT is an incorporated system consisting of silt trap as well as gross pollutant trap, which is capable to trap bed load as well as removal solid waste from stormwater. Basically, it consists of two compartments namely on-line trap and off-line trap as shown in Figure 5.

The on-line trap is referred to a sediment trap combined with a primary trash rack, which is constructed inline with the channel flow direction to treat stormwater at low flow season. It comprises a uniform channel with an expansion extended from the existing drain and with a drop at sediment trap to reduce the flow velocity of the channel. It is essential to slow down the flow velocity because it will increase the settling process for sediment in particular. It is expected at low flows, turbulence is not significant and the pollutant will remains in its original characteristics. Consequently, pollutant that is denser than water will settle or sink at the bottom of the sediment trap, whilst pollutant less dense than water will be removed from stormwater at the primary trash rack. The on-line trap is designed up to stormwater quality treatment recurrent interval (3 month ARI).

Figure 5. Gross Pollutant Trap Prototype

Off-line traps are provided at the both sides of the on-line traps intends to treat stormwater with rainfall events exceed 3 month ARI to serve the large quantity of stormwater. Besides, the off-line trap also functions as the secondary trap or back up trap, which is responsible to treat any stormwater flow whenever the on-line trap is blocked. Thus, the excess water will overspill sideways into off-line trap, bypass the secondary channel and then retain into the main channel as outflow. Additionally, off-line trap channel is designed using self-cleaning principle. The pollutants will be intercepted by the screen and forced down it by a combination of the momentum of the water and gravity, until it comes to rest in a waste collection bin and allowed to dry out for removal.

The GPT prototype sizing is illustrated in Figure 6 designed with the assumption of a catchment area of 50 hectares located in the state of Penang with 30 minutes time of concentration.

The GPT prototype (Figure 7) was constructed at The Hydraulic Lab, School of Civil Engineering, USM. Its operation in trapping the sediment and gross pollutants such floating materials was successfully conducted (Figure 8 & Figure 9).